Method of regenerating deactivated catalysts

ABSTRACT

A deactivated reforming catalyst comprising a type L zeolite containing a Group VIII noble metal may be regenerated and have enhanced dispersion by a method involving contacting the catalyst with oxygen and water at elevated temperatures, contacting the catalyst at elevated temperatures with a source of chlorine such as HCl or Cl 2 , and preferably oxygen and water, contacting the catalyst at elevated temperatures with oxygen and optionally water, and contacting the catalyst at elevated temperatures with hydrogen and optionally water to reduce the catalyst. Preferably the noble metal is platinum.

This application is a continuation of application Ser. No. 08/104,255,filed Aug. 10, 1993, now abandoned, which is a continuation ofapplication Ser. No. 07/709,154, filed Jun. 3, 1991, now abandoned,which is a continuation of application Ser. No. 07/432,221, filed Nov.6, 1989, now abandoned, which is a continuation of application Ser. No.07/205,567, filed Jun. 15, 1988, now U.S. Pat. No. 4,925,819, which is acontinuation of application Ser. No. 06/814,027, filed Dec. 23, 1985,now abandoned, which is a continuation of application Ser. No.06/550,951, filed Nov. 10, 1983, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of regenerating and enhancing thedispersion of moderately or severely deactivated reforming catalystsconsisting of one or more Group VIII noble metals supported on zeolites,preferably a catalyst consisting of platinum on potassium-exchangedzeolite L. The regenerated catalyst herein exhibits improved activityand activity maintenance for light naphtha aromatization over thenon-treated material.

2. Discussion of Relevant References

Several materials have been employed as hydrocarbon conversion catalystsin such processes as reforming, catalytic dewaxing, alkylation,oxidation and hydrocracking. Examples of catalysts useful for thispurpose include those materials comprising a catalytically active metalsuch as a Group VIII noble metal and optionally rhenium supported on orimpregnated into a carrier.

Among the hydrocarbon conversion processes, catalytic reforming in thepresence of hydrogen is one of the most important. Catalytic reformingis a refinery process designed to increase the octane number of naphtha.Typically in this process, the naphtha is passed over a suitablecatalyst under reforming conditions, for example elevated temperaturesand pressures well known in the industry in the presence of hydrogen gasand a H₂ /hydrocarbon mole ratio of about 2 to 20. This process involvesseveral different types of reactions, including isomerization,dehydrocyclization of paraffins to produce naphthenes and aromatics,dehydrogenation of cyclohexanes and other naphthenes and alkanes,isomerization/dehydrogenation of cyclopentanes, isomerization of normalparaffins to isoparaffins, and hydrocracking. Paraffin isomerizationoccurs relatively easily, but contributes only a limited improvement inoctane number. The reforming reactions most important for the productionof high octane components are those which produce aromatics.

The ideal reaction scheme minimizes the hydrocracking of long chainparaffins to gaseous hydrocarbons such as methane and ethane to improvethe yield and selectivity to more valuable products of the otherreforming reactions, particularly dehydrocyclization. Examples of Knowncatalysts useful for reforming include platinum and optionally rheniumor iridium on an alumina support, platinum on type X and Y zeolites,provided the reactants and products are sufficiently small to flowthrough the pores of the zeolites, and platinum on cation exchanged typeL zeolites.

While zeolite L catalysts, usually in their hydrogen form, have beenemployed as catalytic dewaxing catalysts and in other applications, theyare, particularly useful in reforming because they decrease the amountof hydrocracking which occurs during reforming. For example, U.S. Pat.No. 4,104,320 discloses that the use of zeolite L as a support increasesthe selectivity of the reaction for producing aromatic products. Thisimprovement, however, has been made at the expense of catalyst life.U.K. Appln. 82-14147 filed May 14, 1982 to Wortel entitled "ImprovedZeolite L" teaches that a highly crystalline zeolite L material having acylindrical morphology leads to an improved catalyst life fordehydrocyclization reactions over a conventionally prepared zeolite Ldisclosed in U.S. Pat. No. 3,216,789. Finally, Belg. Pat. Nos. 895,778and 895,779 disclose use of a barium-exchanged zeolite L catalyst forhigh yields in reforming, dehydrocyclization, dealkylation anddehydroisomerization.

Because reforming catalysts tend to deactivate on prolonged use thereofdue to the buildup of coke deposits; regeneration becomes necessary toprolong the life of the catalyst. In addition, platinum supported onzeolite L experiences an agglomeration of the platinum particles so asadversely to affect catalyst activity. Thus, for the latter catalysteffective regeneration requires not only the removal of carbon-aceousresidue from the surface of the catalyst, but also the redispersion ofthe platinum component of the catalyst.

It is well known that coke deposits may be removed from such catalystsby heating them in the presence of dilute oxygen at a flame-fronttemperature of 430° to 540° C. This combustion may be preceeded by aflushing with hydrogen or nitrogen gas. High temperature decoking leads,however, to loss of surface area of the supported metal particles and toremoval of platinum from the zeolite channels, thus resulting in loss ofcatalyst activity. Thus, after combustion, the catalyst is oftensubjected to oxychlorination by contact with air and chlorine or achlorinated compound such as CCl₄ at elevated temperatures. FrenchPatent Publication 2,360,540 filed Sep. 9, 1981 to Bernard et al.further teaches that catalyst regeneration is improved by subjecting thecatalyst after oxychlorination to a treatment with water and cooling airbefore the catalyst is reduced. In addition, French. Appl. No. 8,000,114to Bernard discloses a hydrogen regeneration technique.

Not all of the known regeneration techniques, however, effectivelyregenerate the catalyst, particularly if the catalyst is severelydeactivated. Redispersion of agglomerated platinum particles in such acatalyst, where the particles are of comparable size to the main zeolitechannel, is difficult due to inhibited transport of the reactive gasesused in the redispersion.

SUMMARY OF THE INVENTION

In accordance with the present invention it has now been found thatdeactivated reforming catalysts may be effectively regenerated toenhance their catalytic activity and activity maintenance properties bya process whereby coke deposits are removed from the catalyst, thedispersion of the metal is enhanced by an oxychlorination procedure. Theprocedure leads temporarily to excess chlorine on the catalyst surface.The excess chlorine is then removed from the catalyst and the noblemetal is stabilized on the catalyst surface. More preferably the methodherein for regenerating deactivated reforming catalysts which comprise atype L zeolite containing at least one Group VIII noble metal comprisesthe steps of:

(a) heating the catalyst at a temperature of from about 380° to 540° C.in the presence of a gaseous stream comprising oxygen, hydrogen or aninert gas and from 0 to 10% by volume water for up to 10 hours;

(b) This step can proceed in one of four manners:

(i) The catalyst is oxychlorinated (dispersion of the noble metal) byheating the catalyst at a temperature of from about 400 to 530° C. forup to 10 hours in the presence of a gaseous stream comprising from 0 to10% by volume water and a source of chlorine in the presence of oxygen.

(ii) The catalyst is chlorinated by heating the catalyst at atemperature from about 400° to 530° C. for up to 10 hours in thepresence of a gaseous stream comprising from 0 to 10% by volume waterand a source of chlorine in the presence of hydrogen, but when HCl isthe source of chlorine, hydrogen is not required. Oxychlorination willthen occur in the subsequent step (c).

(iii) Combining steps (ii) and (i) in that order. The preferred chlorinesources in these steps are HCl and Cl₂.

(iv) The catalyst is chlorinated by heating at a temperature from about400° to 530° C. for up to 10 hours in the presence of a gaseous streamcomprising from 0 to 10% by volume of water and an effective amount ofchlorine.

In addition, if the catalyst is deactivated and hydrogen or an inert gasis employed in the first step, oxygen must be present in the gaseousstream of the second step, i.e., this second step must be anoxychlorination step to ensure coke removal.

(c) heating the catalyst at a temperature of from about 400° to 540° C.for up to 7 hours in the presence of a gaseous stream comprising oxygenand essentially no water, or for up to 5 hours in the presence of agaseous stream comprising oxygen and from greater than 0 to 10% byvolume water; and

(d) heating the catalyst at a temperature of from about 400° to 530° C.for up to 10 hours in the presence of a gaseous stream comprising from 0to 10% by volume water and a source of hydrogen.

The preferred catalysts prepared by the process herein consist of a typeL zeolite having exchangeable cations of which at least 75 percent areselected from Li, Na, K, Rb, Cs, Ca and Ba cations which contain atleast one Group VIII noble metal and are characterized in that theparticles of the noble metal, prior to reduction thereof, are welldispersed over the surface of the catalyst and at least about 90% of thenoble metal, prior to reduction, is dispersed in the form of particleshaving a diameter of less than about 7 Å. More preferably the catalystwill be such that more than 98% of the noble metal, prior to or afterreduction, is dispersed in the form of particles having a diameter lessthan about 7 Å and less than 2% is dispersed in the form of particleswith a diameter of 7 Å or greater.

The method herein results in catalysts which have effective catalystactivity and activity maintenance for a sustained period of reforming.This method is particularly advantageous for regenerating severelydeactivated catalysts because it improves redispersibility by opening upthe zeolite channels which are clogged by agglomerates of the noblemetal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a plot of the selectivity for benzene as measured bythe selectivity parameter (weight ratio of benzene product to the sum ofbenzene plus C₂ to C₅ products) achieved, as a function of the terminalcracking index (TCI) defined hereinbelow, for platinum on type Lzeolites and for platinum on silica after a time on oil of either about3 or about 22 hours.

FIG. 2 represents an electron micrograph of a deactivated catalyst whichwas treated at 510° C. and 700 kPa (gauge) for 350 hours at a spacevelocity of 2.5 w/w/hour and a H₂ :hydrocarbon molar ratio of 6, whereinthe hydrocarbon feed was 60% n-hexane, 30% isohexane and 10%methylcyclopentane, where the metric scale is indicated on themicrograph.

FIG. 3 represents an electron micrograph of the catalyst of FIG. 2 afterregeneration and dispersion by the process of this invention, where themetric scale is indicated on the micrograph.

FIG. 4 represents an electron micrograph of a comparative catalystregenerated and dispersed without an oxygen post-treat step, where themetric scale is indicated on the micrograph.

FIG. 5 represents an electron micrograph of a catalyst regenerated anddispersed using a dry process, where the metric scale is indicated onthe micrograph.

FIG. 6 represents an;electron micrograph showing a Z-contrast image of acatalyst of this invention.

FIG. 7 represents an EXAFS pattern for a platinum on potassium-exchangedzeolite L catalyst.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The type L zeolites are defined herein as synthetic zeolites whichcrystallize in the hexagonal system with a characteristic x-raydiffraction pattern obtained from CuKα radiation with the major d(Å)peak values set out in Table A:

TABLE A

16.1±0.3

7.52±0.04

6.00±0.04

4.57±0.04

4.35±0.04

3.91±0.02

3.47±0.02

3.28±0.02

3.17±0.01

3.07±0.01

2.91±0.01

2.65±0.01

2.46±0.01

2.42±0.01

2.19±0.01

Thus, they share a common zeolitic structure. For purposes of thisinvention type L zeolites have a general formula as follows:

    0.9-1.3M.sub.2/n O:Al.sub.2 O.sub.3 :x SiO.sub.2 :yH.sub.2 O

where M designates at least one exchangeable cation, n represents thevalence of M, y is any value from 0 to about 9, and x is disclosed inU.S. Pat. No. 3,216,789 to be 5.2 to 6.9, but may be outside this rangeprovided the x-ray diffraction pattern of the zeolite is the same aszeolite L. A more complete description of zeolite L is provided in U.S.Pat. No. 3,216,789. Zeolite L has channel-shaped pores undulating fromabout 7 to 13 Å in diameter and may occur in the form of cylindricalcrystals with a mean diameter of at least 0.5 micron and an aspect ratioof at least 0.5 (as described, e.g., in U.K. Appln. 82-14747, the entiredisclosure of which is incorporated herein by reference), as well as inother sizes and shapes.

The type L zeolites are conventionally prepared such that M in the aboveformula is potassium. See, e.g., U.S. Pat. Nos. 3,216,789 and 3,867,512.The potassium can be ion exchanged, as is well known, by treating thezeolite in an aqueous solution containing other cat ions. It isdifficult, however, to exchange more than 75% of the original potassiumcations, because some cations occupy sites in the zeolite structurewhich are nearly inaccessible. At least 75% of the exchangeable cationsare selected from lithium, sodium, potassium, rubidium, cesium, calciumand barium. More preferably, the cation is sodium, potassium, rubidiumor cesium, more preferably still, potassium, rubidium or cesium, andmost preferably potassium. Optionally, the exchangeable cations mayconsist of mixtures of the above-named Group IA cations or mixtures of aGroup IA/cation and barium or calcium cations. These mixtures of cationsmay be achieved, for example, by treating the zeolite L with an aqueoussolution containing, e.g., a rubidium and/or cesium salt and thenwashing to remove excess ions. This ion exchange treatment can berepeated to effect further ion exchange, although to a lesser degree.

The Group VIII noble metals which are necessary for catalytic activityare those metals from Group VIII of the Periodic Table of Elements whichare selected from osmium, ruthenium, rhodium, iridium, palladium andplatinum. Preferably, the metals which are employed herein are platinum,rhodium or iridium, and most preferably platinum. The metals may be bepresent in any combination desired. Rhenium, a Group VIIB metal, mayalso be present so long as at least one Group VIII noble metal ispresent.

The amount of Group VIII noble metal present in the catalyst will be aneffective amount and will depend, for example, on required catalystactivity, ease of uniform dispersion, and the crystal size of the type Lzeolite. Crystal size limits the effective catalyst loading since highlyloaded crystals of zeolite which have a large dimension parallel to thechannels could easily lead to pore plugging during operation as thenoble metal agglomerates inside the channels. Generally however, thelevel of metal present will range from about 0.1 to 6%, preferably 0.1to 3.5% and more preferably 0.1 to 2.5% by weight of the catalyst.Furthermore, the amount of metal present is generally from about 0.1 to2.0% by weight of the catalyst if the average zeolite crystallite sizeparallel to the channels is greater than about 0.2 micron, and fromabout 1.0 to 6% by weight if the average zeolite crystallite sizeparallel to the channels is no greater than about 0.2 micron.

The Group VIII noble metals may be introduced into the zeolite by, forexample, ion exchange, impregnation, carbonyl decomposition, adsorptionfrom the gaseous phase, introduction during zeolite synthesis, andadsorption of metal vapor. The preferred technique is ion exchange. Insome cases, e.g., when the metal(s) have been introduced by anion-exchange process, it is preferred to remove the residual acidity ofthe zeolite by treating the catalyst, which has previously been reducedby hydrogen, with an aqueous solution of an alkaline base such aspotassium carbonate. This treatment will neutralize any hydrogen ionsformed during the reduction of Group VIII noble metal ions by hydrogenand will adjust the pH to within the range specified hereinbelow.

The preferred regenerated and dispersed reforming catalyst produced bythe process is a unique composition characterized by several propertieswhich lead to improved selectivity and activity maintenance over what isexhibited by other reforming catalysts outside the class defined hereinunder the same catalytic conditions. The greater the degree ofdispersion of the metal within the channels, i.e., onto the internalsurface area of the zeolite, the better will be the activity maintenanceof the catalyst.

The activity of a catalyst is a measure of its ability to convert feedto products. While a catalyst may have a high activity, the productsformed may not be necessarily the desired products. The term"selectivity" is a measure of the ability of the catalyst to convertfeed to desired products. Activity maintenance concerns the ability ofthe catalyst to maintain a portion of its activity over time atconversion conditions, other variables remaining constant.

The decline in catalyst activity at conversion conditions is believed tobe due primarily to crystal growth or agglomeration of the noble metalparticles and secondarily to the formation of coke on the external andinternal surfaces of the catalyst. At the same noble metal loading,catalysts containing particles or crystals of noble metals of a sizegreater than that disclosed above are less active and less selectivethan catalysts containing the smaller particles. Coke formation,probably due to complex condensation and dehydrogenation reactions,results in the shielding of the noble metal from the reaction mixture,thereby limiting the catalytic sites available for promoting reactions.

As catalytic activity declines by virtue of agglomeration and cokeformation the yield of desirable products will decline, and dependingupon the economics of the operation a process employing the catalystwill have to be interrupted and catalyst activity restored to itsinitial value. Generally, catalytic activity can be maintained byincreasing temperature, but there are limits beyond which thetemperature cannot be increased, for example, temperatures which willchange the nature of the zeolite or lead to undesirable side reactions.

Catalyst activity will decline over time as severity increases. Factorswhich affect severity include: the hydrogen to oil mole ratio, hydrogenpartial pressure, total pressure, temperature, feed rate per volume ofcatalyst (space velocity), and type of hydrocarbon in the feed.

In the measurement of activity maintenance all variables are fixed andonly the catalyst differs. Thus, an activity of one catalyst over aperiod of time can be directly compared to the activity of anothercatalyst over the same time period where feed, hydrogen to oil ratios,pressures, etc. are constant.

Catalysts may be evaluated for their activity maintenance by two tests.In the Standard Activity Test (SAT) which is conventionally employed,the catalyst is sieved, mixed with sieved silica and charged into areactor. The catalyst is then subjected to conditions of 510° C., 700kPa (gauge), a space velocity of 2.5 w/w/hour and a H₂ /hydrocarbon moleratio of 6. The feed consists by weight of 60% n-hexane, 30%methylpentane and 10% methylcyclopentane (MCP). Catalysts are evaluatedwith respect to their cycle lengths, defined as the number of hours inwhich a time-averaged benzene yield of 50% by weight is maintained.

A second test of activity maintenance known as the Expedited CatalystAging Test (ECAT) has been developed where a total of 0.20 g of catalystconsisting of 20/40 mesh particles is mixed with 0.80 g of SiO₂ of 20/40mesh. The resulting catalyst charge is introduced into a stainless steeldownflow reactor of about 1 cm inner diameter equipped with athermo-couple. Before introduction of feed the catalyst is reduced insitu under flowing H₂ gas at temperatures no greater than 525° C. Afterreduction the feed introduced into the reactor along with H₂ gas at amole ratio of H₂ :hydrocarbon of 6 and a space velocity of 50 w/w/hour,at a temperature of 510° C. and a pressure of 700 kPa (gauge). Theproducts were analyzed using on-line gas chromatography, although othertechniques are available and known in the industry. Catalysts areevaluated with respect to their benzene yield (weight percent) after 24hours on feed.

The improvement in activity maintenance is manifested by the preferredcatalysts of this invention in that they all provide a benzene yield inexcess of 7 weight percent after 24 hours on oil in the ECAT test asdescribed above using a feed comprising 20% by volume methylcyclopentaneand 80% by volume n-hexane.

These preferred catalysts of enhanced dispersion, at least prior toreduction thereof, consist of noble metal particles well dispersed overthe surface of the catalyst. By the expression "well dispersed over thecatalyst surface" is meant that the diameter of substantially all theparticles in Which the noble metal is dispersed is less than 7 Å, asdescribed hereinbelow. By "substantially all" is meant that at least90%, preferably 98%, of the noble metal is dispersed in the form ofsmaller than 7 Å particles. Initial dispersion of the catalyst is foundto correlate directly with its activity maintenance when it is subjectedto reforming conditions.

The surface of the catalyst over which the particles are dispersednecessarily includes the internal surface of the catalyst, i.e., thechannels of the zeolite, but may also include dispersion over theexternal surface, which is considerably smaller. The dispersion will bepresent on those surfaces of the catalyst which are accessible to noblemetal loading and dispersion techniques. The most preferred catalystwill contain highly dispersed noble metal atoms, all or substantiallyall of which are located inside the pores of the catalyst rather than onthe exterior surface thereof. Thus, at least 75% and preferably, atleast 90%, of the noble metal will exist inside the channels of thepreferred catalyst herein. Further, the noble metal inside the channelsmust be well dispersed. The location of the particles can be inferredfrom electron spectroscopy for chemical analysis (ESCA) measurementswhich are well known to those skilled in the art.

ESCA measurements made to determine the possible preferentialdisposition of platinum on the external surface of the catalyst of thepresent invention showed no substantial platinum accumulation on theexternal zeolite surface of the freshly prepared catalyst or on thecatalyst which has been treated by the multi-step technique as describedhereinbelow. However, after the catalyst was deactivated, and after thedecoking step which comprised an oxygen burn in the presence of watervapor, appreciable agglomeration of platinum near or at the externalzeolite surfaces was detected. This is shown in the following tablewhich gives the intensity ratio of the platinum 4f peak to the silicon2p peak of the zeolite support in the ESCA analysis, which in turn is anindication of surface platinum-to-silicon ratio in the material beingstudied.

    ______________________________________                                        ESCA Pt.sub.4f /Si.sub.2p Intensity Ratio of Potassium-                       Exchanged Zeolite L Catalyst of this Invention                                ______________________________________                                        Fresh Catalyst C of Example 1                                                                      0.038                                                    (ion exchanged, 350° C. calcined)                                      Deactivated Catalyst C                                                                             0.052                                                    Wet Oxygen Decoked Catalyst C                                                                      0.12                                                     Catalyst of Enhanced Dispersion                                                                    0.034                                                    Prepared by Multi-step Method                                                 from Deactivated Catalysts                                                    ______________________________________                                    

These data support the micrograph evidence, showing that in freshlyprepared catalysts and catalysts which are treated by the multi-stepmethod, the platinum was not preferentially concentrated near or at theexternal surfaces.

The property that the particles be well dispersed over the surface ofthe catalyst implies that there is no preferential disposition of noblemetal particles on the external surface. In other words, it signifiesthat the noble metal is substantially uniformly distributed throughoutthe internal and external surfaces of the catalyst. This uniformdistribution is approached to the extent that the ESCA measurements ofthe catalyst indicate substantially no preferential disposition of thenoble metal on the external surface of the zeolite.

Additionally, the preferred catalyst produced by the process herein ischaracterized in that at least about 90% of the noble metal prior toreduction thereof is dispersed in the form of particles having adiameter less than about 7 Å. Thus, if, for example, more than 10% ofthe noble metal is dispersed in the form of particles measured to havediameters of 12-15 Å, or if more than 10% of the noble metal isdispersed in the form of particles measured to have diameters of greaterthan 15 Å, the catalyst exhibits reduced activity and activitymaintenance. As measured by conventional bright field imaging in atransmission electron microscope with 4 Å point-to-point resolution, theparticle size in the most preferred catalyst herein is found to be suchthat no greater than 2% of the noble metal is dispersed in the form ofparticles measured to be about 7 Å or greater in diameter.

The noble metal atoms may be clustered in any type of configuration,including that in which the atoms are connected to form a raft shapewhere they exist as a monatomic layer. The size of the particles asdescribed above may be measured using high resolution electronmicroscopy. In this procedure, described in P. C. Flynn et al., J.Catal., 33, 233-248 (1974), the noble metal loaded zeolite is preparedfor the transmission electron microscope (TEM) by crushing in an agatemortar and pestle to produce zeolite fragments through which an electronbeam can pass. The crushed powder is ultrasonically dispersed in ethanoland a drop of this suspension is allowed to dry onto a standard 3 mm TEMgrid, which is covered with a thin (≦200 Å) amorphous carbon film.Samples are analyzed in a Philips 400T TEM at 100 KV by conventionalbright field imaging. Owing to the complexities of the contrast andimaging process involved in the bright field imaging mode, the lowestmeasurable noble metal particle diameter is 7 Å when the particle is inraft shape, and is 5 Å, when the particle is in spheroidal(agglomerated) shape. The actual size may differ by ±2 Å from thesemeasurements. Therefore, noble metal raft particles less than 7 Å indiameter in a catalyst of good dispersion cannot be detected by thebright field imaging method using the Philips 400T microscope. Thus,degree of dispersion is determined by measuring the quantity of noblemetal dispersed in measurable particles of diameter 7 Å or greater. Theremainder of the noble metal necessarily exists in the form of particlesbelow 7 Å in diameter.

When different samples are compared under the electron microscope, thereis a ±50% uncertainty in the relative thickness of the specimen.Consequently, the estimated percentage amount of visible particulates (7Å and greater if rafts, 5 Å and greater if spheroids) is subject to thissame ±50% uncertainty. For example, a catalyst reported as comprising10% noble metal particles measuring 7 Å in diameter or greater and 90%below 7 Å in diameter could actually consist of between 5% and 15% ofvisible particles measuring 7 Å or greater in diameter and between 95and 85% of highly dispersed clusters below 7 Å in diameter.

Samples of fresh catalysts may also be examined using Z-contrastimaging. In this method samples are prepared exactly as for bright fieldimaging, but are examined using a high resolution scanning transmissionelectron microscope (STEM). In this instrument samples are examined by afinely focused electron beam about 5 Å in diameter, which is scannedacross the sample. Signals collected by the various detectors positionedaround the sample can be displayed on a synchronously scanned TV monitorto build up the image. Images formed by taking the ratio of the annulardetector signal and the transmitted electron image-loss signal showcontrast which is sensitive to variations of atomic number Z within thesample. Pt has an atomic number Z equal to 78 whereas zeolite Lcomprises only low Z atoms, K, Si, Al, O, H (Z=19, 14, 13, 8 and 1,respectively). Thus, Z-contrast imaging provides a sensitive method ofdetecting very small Pt clusters when supported on the zeolite L.

FIG. 6 is a Z-contrast image of a thin edge of catalytic material. Onthe micrograph, some of the visible small clusters of platinum, probably3 atoms or less, are indicated by arrows. Other particles, some largerin size, are also visible. Also visible is the 16 Å spacing of thechannels of the zeolite substrate. The metric scale on this diagram(horizontal line) corresponds to 50 Å. The Z-contrast image shows thatthe more highly active fresh catalyst, i.e. catalyst before exposure tohydrocarbon feed, contains many Pt clusters smaller than 5 Å, comprising5 atoms or fewer. In contrast, a deactivated catalyst comprises mostly12 to 15 Å Pt agglomerates and few clusters containing less than 5atoms. Catalysts which are regenerated and dispersed also show a fine Ptdispersion with many clusters containing 5 atoms or fewer.

An additional method of probing the extent of dispersion and chemicalstate of the noble metal particles is EXAFS (Extended X-ray AbsorptionFine Structure). EXAFS is an element-specific electron scatteringtechnique in which a core electron ejected by an x-ray photon Probes thelocal environment of the absorbing atom. The ejected photoelectron isback-scattered by the neighboring atoms of the absorbing species andinterferes constructively or destructively with the outgoing electronwave, depending on the energy of the photoelectron. The energy of thephotoelectron is equal to the difference between the x-ray photon energyand a threshold energy associated with ejection of the electron. In theEXAFS experiment the photoelectron energy is varied by varying theenergy of the incident x-ray beam. The interference between outgoing andbackscattered electron waves as a function of energy modulates the x-rayabsorption coefficient so that the EXAFS function k·x (k) is observedexperimentally as oscillations in the absorption coefficient u on thehigh energy side of the

For the EXAFS experiments, samples of various catalysts were dried,calcined and subsequently reduced in situ at between 400° and 500° C.The EXAFS measurements were made in flowing hydrogen at about -170° C.FIG. 7 shows the EXAFS pattern for a platinum on potassium-exchangedzeolite L catalyst similar to catalyst C of Example 1. The followingtable gives the nearest neighbor coordination number and interatomicdistances calculated from the EXAFS data for various samples. Comparisonof potassium-exchanged zeolite L with 1% Pt on SiO₂ (sintered to berepresentative of bulk platinum metal) shows that the Pt--Pt distanceremains substantially unchanged from that in bulk metal whereas thePt--Pt coordination is reduced from the bulk value of 12 nearestneighbors to approximately 5. This value of 5 is consistent with acatalyst in which the Pt is highly dispersed. For example, 1% Pt/Al₂ O₃described in Via et al., J. Chem. Phys., 71, 690 (1979) has acoordination number of about 7 and gives a hydrogen to metal atom ratioof about 1 in a standard chemisorption test. These results indicate thatessentially all of the metal atoms are surface atoms.

The EXAFS data for catalyst H of Example 1, which was deactivated on oiland regenerated via wet coke burn, oxychlorination, and wetpost-treatment, is qualitatively the same as that obtained for the freshreduced catalyst described above and is thus consistent with a highdegree of metal dispersion.

    ______________________________________                                        EXAFS Properties of Supported Pt Catalysts                                    Catalyst            N.sub.1 R.sub.1 (Å)                                   ______________________________________                                        1% Pt/SiO.sub.2, sintered                                                                         12      2.775                                             0.6% Pt/potassium-exchanged                                                                       5 ± 1.5                                                                            2.766                                             zeolite L                                                                     1% Pt/Al.sub.2 O.sub.3                                                                            7 ± 1.5                                                                            2.758                                             ______________________________________                                         Note:                                                                         N.sub.1 is equal to average nearest neighbor Pt--Pt coordination number.      R.sub.1 is equal to average nearest neighbor Pt--Pt interatomic distance.

As an additional characteristic, if the catalyst is loaded with thenoble metal by, for example, an ion-exchange technique, it is desirablethat the type L zeolite chosen as the support have, prior to beingloaded with the noble metal, a pH between 9 and 11.5 as determined bythe following method: A 2.5 g sample of zeolite is equilibrated atambient temperature with 70 cc of distilled water and the pH of theslurry is determined. The pH should be higher than about 9.3, preferably10 to 11. If the pH of the zeolite is not within the above range, webelieve that traditional impregnation or ion exchange methods will notyield a catalyst which has highly dispersed noble metal particles withina preferred size range. While not limited to any one theory, the beliefis that the pH of the zeolite characterizes a surface state of thezeolite and controls the affinity of the zeolite for the noble metalduring ion exchange or impregnation.

Another feature of the preferred catalyst produced herein whichidentifies those catalysts having the improved selectivity and activitymaintenance as defined herein is its terminal cracking index (TCI). Thisindex is defined as the molar ratio of pentanes to butanes produced whenthe catalyst is evaluated by the ECAT procedure described above using100% n-hexane feed. The index measures the degree to which terminalcracking is promoted as opposed to internal cracking for a givencatalyst. The higher this index the more selective the catalyst hereintoward aromatization products because increased terminal crackingrelative to internal carbon cracking of the hydrocarbon chain asmeasured by reforming n-hexane indicates that terminal adsorption of thesubstrate onto the noble metal occurs preferentially, as opposed tointernal adsorption, thus favoring, for example, 1-6-closures foraromatization and terminal cracking of the hexane feed.

This index shows not only that to perform well the preferred catalystherein must not be acidic, but also that the noble metal is not to anysignificant extent on the exterior surface of the catalyst, but ratheris inside the channels of the catalyst and adsorbs the terminal carbonatom of straight-chain paraffins. For purposes herein, the terminalcracking index of the preferred type L zeolite catalyst produced by theprocess of this invention is greater than about 1.5, preferably greaterthan about 1.7.

FIG. 1 illustrates the relationship between the selectivity parameter(defined as the weight ratio of benzene product to the sum of totalbenzene plus C₂ -C₅ produced) and the terminal cracking index (TCI) forzeolite L loaded with 0.6% Pt. The relationship shows a clearcorrelation, indicating that the terminal cracking index does accuratelyidentify the selective reforming catalysts when a type L zeolite isemployed as the catalyst base. FIG. 1 also illustrates that aconventional active 0.6% Pt on silica catalyst has a much lower terminalcracking index which does not exceed 1.0. Such a value is typical ofrandom cracking of a hexane molecule.

The TCI of a given catalyst varies with the type of feed and theseverity of the conversion. Generally, as the result of secondarycracking reactions, the TCI for catalysts which have a TCI above oneunder ECAT conditions described above will decrease as the conversion isincreased, as indicated in the following Table I. These results showthat ECAT is a better test for determining the TCI than the SAT testbecause the TCI is not meaningful at the high conversion levelsencountered under SAT test conditions.

                  TABLE I                                                         ______________________________________                                        0.6 wt. % Pt on Zeolite L Catalyst                                            Loaded by Ion Exchange                                                                     H.sub.2 /Hydro-                                                                      After 1-3 Hours on Oil                                          Pres-   Space    carbon Benzene                                                                              Conver-                                  Temp. sure    Velocity (mole: Yield  sion                                     (°C.)                                                                        (KPa)   (w/w/hr.)                                                                              mole)  (wt. %)                                                                              (wt. %)                                                                             TCI                                ______________________________________                                        510   700     50       6.0    25.1   31.7  1.57.sup.(1)                       510   700     2.33     6.4    49.0   97.5  0.82.sup.(2)                       ______________________________________                                         .sup.(1) This run was made using 100% nhexane feed.                           .sup.(2) This run was made using a feed of 60% nhexane, 30% methylpentane     and 10% methylcyclopentane. The latter two feed constituents are known to     raise the TCI of the catalyst above what it would be using 100% nhexane       feed.                                                                    

The TCI also varies with the type of catalyst employed, as indicated inTable II.

                  TABLE II                                                        ______________________________________                                                               Benzene Yield                                                         wt. %   at 1-3 hrs.                                            Catalyst       Pt      (wt. %)     TCI                                        ______________________________________                                        Pt on NaX Zeolite                                                                            1.0     30          0.9                                        Pt on (Na/K) X Zeolites                                                                      0.6     19.6        1.2-1.3                                    Pt on K-treated SiO.sub.2 /                                                                  0.73    4.1         0.8                                        Al.sub.2 O.sub.3                                                              Pt on SiO.sub.2                                                                              1.1     15.1        0.9                                        Pt on SiO.sub.2                                                                              0.74    10          0.9                                        Pt on potassium-exchanged                                                                    0.6     21.1        1.3-1.4                                    Zeolite L Catalyst of                                                         U.S. Pat. No. 4,104,320                                                       Pt on potassium-                                                                             1.0     56.9        1.5-2.5                                    exchanged Catalyst                                                            preferred for this                                                            invention                                                                     ______________________________________                                    

These results show that the catalyst of this invention which has a highTCI also has a high benzene yield. Experiments have shown, however, thatthe correlation between TCI and benzene yield cannot be made when thereforming catalyst is much less catalytically active than the catalystslisted in Table II above such as, e.g., platinum supported onKOH-treated (K-treated) silica/alumina.

Catalysts based on type L zeolites which have the preferred noble metalparticle size and dispersion as described above will necessarily havethe preferred terminal cracking index.

The preferred catalysts of this invention also exhibit substantialmonofunctionality. Many known reforming catalysts exhibitbifunctionality in that not only do the noble metal atoms participate inthe catalytic reaction, but also the acidic sites of the catalystsupport. For example, a conventional catalyst consisting of Pt-Re metalson alumina aromatizes hexane through both the metal sites and the acidsites of the alumina support. In contrast, the catalyst herein behavesessentially monofunctionally in that the catalytic reactionspredominantly occur on the noble metal sites of the catalyst, with onlya small amount of catalysis occurring on the acidic sites initiallyproduced when the catalyst is first reduced.

The catalysts herein may be prepared by any of the methods typicallyemployed for preparing noble metal on zeolite catalysts. Noble metalloading, for example, may be carried out by impregnation or ion exchangeas described, e.g., in U.S. Pat. No. 4,104,320.

The noble metal loaded zeolite catalyst thus prepared may be mixed witha binder material and formed into shapes before it is subjected toreforming conditions to improve the resistance of the catalyst totemperature, pressure and attrition. Platinum may be deposited on thezeolite powder before the addition of the binder and the pellet-formingsteps or may be deposited on the formed tablets, pellets or extrudates.Any conventional binder which is substantially inert under theconditions in which the catalyst is to be employed may be utilized.Examples of suitable binders include kaolin, alumina and silica.

The regeneration process herein applies to deactivated catalysts at anystage of deactivation, including those which are severely deactivated.The severely deactivated Pt zeolite L catalysts, for example, haveagglomerated Pt particle sizes comparable to the size of the mainzeolite channel so that redispersion of the agglomerates is madeparticularly difficult. The process herein, however, is also applicableto moderately or mildly deactivated catalysts. Because the moderatelydeactivated Pt zeolite L catalyst has agglomerated Pt particle sizessmaller than the main zeolite channel, removal of Pt agglomerates to theexternal surfaces thereof is not essential for effective dispersion ofthe Pt particles.

After deactivation the catalyst is subjected to the regeneration andenhanced dispersion method of this invention. In this general method,the coke is removed from the deactivated catalyst and the dispersion ofthe noble metal is enhanced by an oxychlorination procedure, forexample, treating the catalyst with a source of chlorine and oxygen. Thecoke removal and oxychlorination steps may be carried out simultaneouslyor as separate steps. The excess chlorine is then removed from thecatalyst and the noble metal therein is stabilized. This is preferablyaccomplished by treating the Catalyst with a gas stream comprising a gasselected from oxygen, steam or mixtures thereof, for example, steam andair, under conditions such that the excess chlorine is removed.Preferably this step is accomplished using a treatment with wet oxygen.Also preferably water is present in the steps.

For use in reforming, the catalyst is reduced under a suitableatmosphere and conditions, preferably as described below.

In the more specific embodiment of this invention described below thegaseous stream used in each step has a remaining percentage (which isnot water, hydrogen, oxygen or a source of chlorine) of an inert gassuch as helium, argon or nitrogen which will not interfere with theprocess. Preferably water is present in the gaseous stream of each step.

In this more specific embodiment, the deactivated catalyst is contactedin a first step with a gaseous stream containing from 0 to 10% by volumewater, preferably 0.5 to 5% by volume water, more preferably 1 to 3%,based on the total stream volume, and oxygen (usually in the form of anoxygen-containing gas such as air), hydrogen or an inert gas at atemperature of from about 380° to 540° C. For purposes herein the inertgas is defined as a gas which does not react with the catalyst and is acarrier for the other gases such as oxygen or hydrogen used in othersteps. Examples of suitable inert gases include helium, argon andnitrogen or mixtures thereof. Preferably, if oxygen is employed, thisfirst step is carried out at two temperatures, the first being at alower temperature range of about 380° to 480° C., and the second beingat a higher temperature range of about 480° to 520° C. The O₂ treatmentat the lower temperature is preferably carried out for a longer timeperiod than the second O₂ treatment. The exact time for heating willdepend on the temperature employed, but generally ranges up to 10 hours,preferably 2 to 8 hours. The amount of oxygen employed is generally from0.1 to 25% by volume, preferably 0.2 to 15% by volume, more preferablyfrom 0.5 to 12% of the gas stream. If hydrogen is employed the amount is1 to 25% by volume, preferably 2 to 20% by volume. If an inert gas isemployed it may constitute up to 100% by volume of the gaseous stream.

The second step of this specific method can proceed in one of fourmanners:

(i) The catalyst is oxychlorinated (dispersion of the noble metal) byheating at a temperature of from about 400° to 530° C., preferably480°-520° C., for up to 10 hours, preferably 1 to 3 hours, in thepresence of a gaseous stream comprising from 0 to 10% by volume water,preferably 0.5 to 5%, more preferably 1 to 3%, a source of chlorine suchas, e.g., Cl₂, HCl, chloroform, methyl chloride, carbon tetrachloride,1,2-dichloroethane, trichloroethane or mixtures thereof, and the like,in an amount of about 0.005 to 10 volume percent, preferably 0.05 to 4%,more preferably 0.05 to 1%, and most preferably 0.05 to 0.5%, and in thepresence of oxygen in an amount of 0.1 to 25% by volume, preferably 0.2to 15%, more preferably 0.5 to 12%.

(ii) The catalyst is chlorinated by proceeding as in (i), except thathydrogen is used in place of oxygen. However, when HCl is the source ofchlorine, hydrogen is not required. When proceeding in this manner,oxychlorination will occur in the third step discussed hereinbelow.

(iii) Steps (ii) and (i) can be combined, in that order the preferredchlorine sources in these steps are HCl and Cl₂.

(iv) The catalyst is chlorinated by heating at a temperature from about400° to 530° C., preferably 480° to 520° C., for up to 10 hourspreferably for 1 to 3 hours in the presence of a gaseous streamcomprising from 0 to 10% by volume of water preferably 0.5 to 5% andmore preferably 1 to 3% and 0.005 to 10% chlorine more preferably 0.05to 1% and most preferably 0.05to 0.5%.

If, however, hydrogen or an inert gas is employed in the first step,oxygen must be present in the gaseous stream of this second step, i.e.,this second step must be an oxychlorination step, to ensure cokeremoval.

In the third step of this specific method (oxygen post-treat step), thecatalyst is contacted with an oxygen-containing gaseous stream (wherethe amount of oxygen in the stream is generally 0.1 to 25% by volume,preferably 0.2 to 15%, more preferably 0.5 to 12% by volume) at atemperature of 400° to 540° C. for up to 7 hours if essentially no wateris employed or for up to 5 hours in the presence of greater than 0 to 10volume percent water. Preferably, this step is carried out from 480° to520° C. in the presence of 0.5 to 5% by volume water for 0.5 to 3 hoursand more preferably in the presence of 1 to 3 volume % water for 0.5 to1.5 hours. If no water is present preferably the step is carried out for0.5 to 6 hours, and more preferably for 1 to 5 hours.

In a fourth and final step (hydrogen reduction step) the catalyst isheated in the presence of a hydrogen-containing gaseous stream (wherethe amount of hydrogen in the stream is generally 1 to 25% by volume,preferably 2 to 20%) in the presence of from 0 to 10% by volume water ata temperature of from about 400° to 530° C. for up to 10 hours.Preferably this final step is carried out at a temperature of 400° to520° C. in the presence of 0.5 to 5% by volume water for 1 to 6 hours.

In all steps, reactor pressure is generally from 0.1 to 2 MPa.Preferably the gas flow rates for each step in the above process rangefrom about 1 to 300 cc/g of catalyst per minute. If no oxychlorinationstep is employed, preferably the gas flow rate of the oxygen post-treatstep is below 20 cc/g catalyst/min. and preferably below 10 cc/gcatalyst/min. Optionally the second step can consist of a chlorinationstep followed by an oxychlorination step as described hereinabove.

The catalysts of enhanced dispersion produced by the process of thisinvention may be employed as reforming catalysts using reformingconditions which are well known in the art.

To minimize any hydrocracking reactions which tend to prevail at thebeginning of the reaction, it may be desirable to introduce a smallamount (less than 0.1% by weight) of sulfur to the catalyst.

The following discussion is particularly directed to the preferredcatalysts produced by the process herein which have the specialproperties defined above. Without being limited to any one theory, it isbelieved that the selectivity and maintenance of the catalyst aregreatly influenced by the manner in which the reactant molecules areadsorbed at the active site of the catalyst. This theory is herein-aftercalled "molecular die" catalysis.

If the hexane is adsorbed terminally (through the C₁ atom), subsequentreaction preferably leads to aromatization. Another reaction occurringto a lesser extent is terminal cracking of adsorbed hexane. If initialadsorption is through a nonterminal carbon atom, no such aromatizationcan occur because end carbon activation is required for 1-6 ringclosure. While terminal cracking also leads to methane production, thearomatization reaction occurs to a greater extent. The adsorptionpattern may be influenced by the three-dimensional structure of thecatalyst, which may columnate the reactant molecules. Furthermore, theinterior structures of and spacial factors within the catalyst mayaffect favorably post-adsorption reactions as by, e.g., stabilizing ordestabilizing the transition states so as to favor cyclization ofadsorbed molecular species.

The channel structure of the zeolites gives rise to these molecular dieeffects, i.e, the zeolite one-dimensional channel structure orients thehexane molecules parallel to the axis of the channels. This orientationfacilitates terminal adsorption onto the noble metal particles, leadingto the observed increased activity and selectivity for aromatization.

The catalysts of enhanced dispersion produced by the process herein neednot be used only in conventional reforming of naphthas derived frompetroleum or other sources of hydrocarbons and boiling in the range ofabout 71° to 216° C., but may also be employed to produce thecorresponding aromatic products from any reactant organic compoundcontaining at least six carbon atoms, including those which containreactant functional groups. Examples of reactant compounds suitable forthis purpose include paraffins such as n-hexane, n-heptane, n-octane,n-nonane, etc., preferably naphtha boiling in the fraction between 71°and 216° C. Preferred reactions herein are aromatization of n-hexane tobenzene, n-heptane to toluene, and n-octane to ethylbenzene and/orxylenes. n-Hexane may be present in the feed or produced byisomerization of methylpentanes and methylcyclopentane. Since thecatalyst is monofunctional and does not promote isomerization withoutcyclization, feed compounds such as dimethylbutanes are not effective.

Additional reactions where the reforming catalyst produced by theprocess herein, especially the preferred class of catalysts producedherein, may be employed advantageously include benzene production fromstreams such as light naphtha, i.e., a naphtha boiling between about 30°and 100° C., high octane gasoline production from naphtha or lightvirgin naphtha where the endpoint is between C₇ and C₁₂, inclusive.

The reforming processes described above are carried out under generalreforming conditions in the presence of hydrogen at a moderate pressureto favor the aromatization reaction thermodynamically. For traditionalreforming of paraffins to aromatics, the temperature depends on theparticular paraffin, but for acceptable rate and selectivity preferablyranges from about 400° to 550° C., more preferably from about 450° to520° C. at pressures of about 200 KPa to 5 MPa, more preferably about500KPa to 4 MPa. If the temperature is much below about 400° C. the yieldof product is quite low, and if the temperature substantially exceedsabout 550° C., other reactions occur which also diminish the yield ofproduct. The liquid hourly space velocity of this reforming reaction ispreferably from about 0.5 to 20 w/w/hour, more preferably from 1 to 10w/w/hour, and the H₂ /reactant mole ratio is preferably from about 2 to20, more preferably from about 4 to 10.

The dehydrocyclization reaction is generally carried out by injecting afeedstock charge in the presence of hydrogen gas into a reactorcontaining the catalyst.

The examples which follow illustrate the efficacy of the invention. Inall examples, parts and percentages are given by weight for solids andliquids and by volume for gas compositions, and temperatures in degreesCentigrade unless otherwise noted.

EXAMPLE 1 Regeneration of Deactivated Catalyst

a. Preparation of Catalyst:

A zeolite L having a composition expressed in moles of pure oxide of:0.99 K₂ O:Al₂ O₃ :6.3SiO₂ :XH₂ O and having a cylindrical shape and anaverage particle size of about 2 to 2.5 micron was prepared by thetechnique described in Example 1 of U.K. Patent Application 82-14147filed May 14, 1982 to Wortel, entitled "Improved Zeolite L." Thus, analkaline synthesis gel was prepared by dissolving 23.40 g of aluminumhydroxide by boiling in an aqueous solution of 51.23 g of potassiumhydroxide pellets (86% pure KOH) in 100.2 g of water to form Solution A.After dissolution any water loss was corrected. A separate solution,solution B, was prepared by diluting 225 g of colloidal silica (Ludox HS40) with 195.0 g of water.

Solutions A and B were mixed for two minutes to form a gel, and justbefore the gel became fully stiff, 224 g thereof was transferred to aTeflon-lined autoclave, preheated to 150° C. and held at thattemperature for 72 hours to bring about crystallization.

b. Platinum Ion-Exchange of Catalyst

The separated zeolite was slurried in water and Pt(NH₃)₄ Cl₂ solutionwas added over about a two-hour period. Following the addition of the Ptsource stirring was continued overnight. The mixture was then filteredand the loaded potassium exchanged zeolite L, containing 0.6% Pt byweight, was dried, tableted, crushed, screened to 20/40 mesh, andcalcined.

c. Deactivation of Catalyst:

This catalyst was then deactivated in a reactor during a light naphthaaromatization run using a feed of 60% n-hexane, 30% isohexane and 10%methyl-cyclopentane, run at 510° C. and 700 kPa (gauge) at a spacevelocity of 2.5 w/w/hour and at a hydrogen to hydrocarbon ratio of 6,for about 350 hours. At the end of the run the deactivated catalyst,called catalyst A, contained 2.18% coke by weight and Pt agglomerates ofabout 12 Å diameter. FIG. 2, representing an electron micrograph ofcatalyst A, shows that 90% of the Pt was dispersed in the form ofparticles measured to have a diameter of about 12 Å and 10% having adiameter less than about 7 Å.

d: Regeneration of Deactivated Catalyst:

A total of 6.2 g of catalyst A was charged in a reactor and contactedwith a series of gas compositions as described below flowing at a rateof 200 cc/min. for the given period of time. The remaining percentage ofthe gas compositions consisted of He gas.

    ______________________________________                                                                            Electron                                                                      Microscopy                                         Temp.   Gas Compo-  Duration                                                                             Measurements                              Step     (°C.)                                                                          position (%)                                                                              (hrs.) After Step                                ______________________________________                                        Wet Coke Burn                                                                          480     2% O.sub.2, 2% H.sub.2 O                                                                  2.5    --                                        Wet Coke Burn                                                                          510     9% O.sub.2, 2% H.sub.2 O                                                                  1.5    >90% of                                                                       Pt 50-150Å                            Wet      510     9.7% O.sub.2, 0.2%                                                                        2.5    >90% of                                   Oxy-             HCl, 2% H.sub.2 O  Pt <7Å                                chlorination                                                                  Wet O.sub.2                                                                            510     9.3% O.sub.2, 2%                                                                          1.0    >99% of                                   Post-Treat       H.sub.2 O          Pt <7Å                                Wet H.sub.2                                                                            510     20% H.sub.2, 2%                                                                           1.4    >99% of                                   Reduction        H.sub.2 O          Pt <7Å                                ______________________________________                                    

The resulting regenerated catalyst B was evaluated and compared withcatalyst A for production of benzene using the ECAT procedure describedabove using a feed of 80% n-hexane and 20% methylcyclopentane. Theresults are indicated below:

    ______________________________________                                                Benzene Yield (wt. %)                                                         Time on Oil (hrs.)                                                    Catalyst  2             24     48                                             ______________________________________                                        Catalyst A                                                                              0.5           --     --                                             Catalyst B                                                                              14.4          12.3   12.0                                           ______________________________________                                    

The terminal cracking index of catalyst B was found to range from 1.77to 2.37 when the feed was 80% n-hexane and 20% methylcyclopentane.

FIG. 3, representing the electron micrograph of this regeneratedcatalyst B, indicates that greater than 99% of the Pt was dispersed inthe form of particles having a diameter less than 7 Å and less than 1%measured to have a diameter of 7 Å or greater.

A micrograph of Catalyst B before the wet H₂ reduction step indicatedthat greater than 99% of the Pt was dispersed in the form of particleshaving a diameter of less than 7 Å and less than 1% of the Pt measuredto be 7 Å or greater. Thus, the reduction step did not adversely affectthe dispersion properties of the catalyst treated by the multi-stepregeneration technique herein.

Interposing a step of cooling to room temperature in the presence ofoxygen or inert gas such as nitrogen for 1 day between the wetoxychlorination and the wet O₂ post treat steps had little effect ondispersion and activity as compared to the above results.

Following the regeneration procedure described above only through thewet oxychlorination step provided a catalyst when recited under ECATconditions with 20% methylcyclopentane and 80% n-hexane feed, yieldedonly 1.0 wt. % benzene after 2 hrs. on oil. When this catalyst wasreduced in hydrogen at 510° C. for one hour, platinum agglomeration isobserved. As shown in FIG. 4, about 40% of the platinum measures about10 Å in diameter with the remainder being dispersed as particles withdiameter less than 7 Å.

When the wet O₂ post-treat of the regeneration and enhanced dispersionprocedure was practiced at 200° C. or lower instead of at about 510° C.,at 2 hrs. on oil the benzene yield was only 2.4 wt. % in ECAT. The aboveprocedure is substantially the same as that described in French PatentPublication 2,360,540.

When the wet O₂ post-treat step was extended from 1 to 7 hrs., ECATyield was 1.3 wt. % benzene at 2 hrs. on oil.

This example illustrates the criticality of the wet O₂ post-treat step.

EXAMPLE 2

A total of 8.5 g of catalyst A of Example 1 was regenerated to givecatalyst C as described in Example 1 using the conditions and stepsgiven below:

    ______________________________________                                                   Temp.    Gas Composition                                                                             Duration                                    Step       (°C.)                                                                           (%)           (hrs.)                                      ______________________________________                                        Dry Coke Burn                                                                            380      2.1% O.sub.2  2.5                                         Dry Coke Burn                                                                            510      10% O.sub.2   1.5                                         Dry        510      9.4% O.sub.2 0.3% HCl                                                                       2.5                                         Oxychlorination                                                               Dry O.sub.2                                                                              510      9.8% O.sub.2  1                                           Post-Treat                                                                    Dry H.sub.2                                                                              510      20% H.sub.2   1                                           Reduction                                                                     ______________________________________                                    

FIG. 5 shows that catalyst C is not as well dispersed as catalyst B inthat 20% of the Pt was dispersed in the form of particles measured asaveraging about 8 Å, and only 80% of the platinum was dispersed inparticles below 7 Å. The activity and activity maintenance of catalyst Cwere satisfactory, but were less than that of catalyst B, as shown bythe ECAT results below obtained using the procedure and feed of Example1.

    ______________________________________                                                     Benzene Yield (wt. %)                                                         Time on Oil (hours)                                              Catalyst       2 hours 24 hours                                               ______________________________________                                        Catalyst C     7.7     8.9                                                    Catalyst B     14.4    12.3                                                   ______________________________________                                    

EXAMPLE 3

A total of 4.2 g of catalyst A of Example 1 was regenerated anddispersed to give catalyst D as described in Example 1 except that theflow rate in the wet O₂ post-treat was 50 cc/min., using the conditionsand steps given below:

    ______________________________________                                                   Temp.    Gas Composition                                                                             Duration                                    Step       (°C.)                                                                           (%)           (hrs.)                                      ______________________________________                                        Wet Coke Burn                                                                            380      2.2% O.sub.2, 2%                                                                            2.5                                                             H.sub.2 O                                                 Wet Coke Burn                                                                            510      9.3% O.sub.2, 2%                                                                            1.5                                                             H.sub.2 O                                                 Wet HCl Treat                                                                            510      0.2% HCl, 2%  1.25                                                            H.sub.2 O                                                 Wet O.sub.2                                                                              510      10% O.sub.2   1                                           Post-Treat                                                                    Wet H.sub.2                                                                              510      20% H.sub.2   1                                           Reduction                                                                     ______________________________________                                    

The ECAT results using 100% n-hexane show that HCl may be employed alonewithout simultaneous introduction of oxygen gas:

    ______________________________________                                               Benzene Yield (wt. %)                                                                       TCI                                                             Hours on oil  Hours on oil                                                      2 hrs.   22-24 hrs. 2 hrs. 22-24 hrs.                                ______________________________________                                        Catalyst D                                                                             16.4     20.1       1.52   1.64                                      ______________________________________                                    

The electron micrograph indicated that greater than 99% of the Pt wasdispersed in the form of particles having a diameter less than 7 Å. Thisexample shows that oxychlorination can take place during the oxygenpost-treatment if the catalyst is chlorinated in the absence of oxygen.

EXAMPLE 4

A total of 4.2 g of catalyst A of Example 1 was regenerated anddispersed to give catalyst E as described in Example 1 using theconditions and steps given below:

    ______________________________________                                                   Temp.    Gas Composition                                                                             Duration                                    Step       (°C.)                                                                           (%)           (hrs.)                                      ______________________________________                                        Wet Coke Burn                                                                            380      2% O.sub.2, 2%                                                                              2.5                                                             H.sub.2 O                                                 Wet Coke Burn                                                                            510      9.3% O.sub.2, 2%                                                                            1.5                                                             H.sub.2 O                                                 Wet        510      9.6% O.sub.2, 2.5                                         Oxychlorination     0.15% Cl.sub.2 O                                          Wet O.sub.2                                                                              510      9.7% O.sub.2, 2%                                                                            1                                           Post-Treat          H.sub.2 O                                                 Wet H.sub.2                                                                              510      20% H.sub.2, 2%                                                                             1                                           Reduction           H.sub.2 O                                                 ______________________________________                                    

The ECAT results using a 100% n-hexane feed showed that catalyst Eexhibited good initial activity and activity maintenance in theproduction of benzene.

    ______________________________________                                                      Benzene Yield (wt. %)                                                         Hours on oil                                                                    2 hrs. 22-24 hrs.                                             ______________________________________                                        Catalyst E      36.0   18.0                                                   ______________________________________                                    

EXAMPLE 5

A total of 4.9 g of catalyst A of Example 1 was regenerated anddispersed to give catalyst F as described in Example 1 using theconditions and steps given below:

    ______________________________________                                                   Temp.    Gas Composition                                                                             Duration                                    Step       (°C.)                                                                           (%)           (hrs.)                                      ______________________________________                                        Wet Coke Burn                                                                            380      2.3% O.sub.2, 2%                                                                            2.5                                                             H.sub.2 O                                                 Wet Coke Burn                                                                            510      10.1% O.sub.2, 2%                                                                           1.5                                                             H.sub.2 O                                                 Wet        510      10% O.sub.2, 0.2%                                                                           2.5                                         Oxychlorination     HCl, 2% H.sub.2 O                                         Dry Helium 510      100% He       1                                           Treat                                                                         Dry O.sub.2                                                                              510      10% O.sub.2   1                                           Post-Treat                                                                    Dry H.sub.2                                                                              510      20% H.sub.2   1                                           Reduction                                                                     ______________________________________                                    

The ECAT results using 20% methylcyclopentane and 80% n-hexane show thatthe dry helium, oxygen and hydrogen steps decreased the performance ofthe catalyst as compared to wet post-treat steps, as shown below. Theinitial activity of catalyst F, however, was satisfactory.

    ______________________________________                                                      Benzene Yield (wt. %)                                                         Time on Oil (hrs.)                                              Catalyst        2 hrs. 22-24 hrs.                                             ______________________________________                                        Catalyst F      11.6   6.1                                                    Catalyst B      14.4   12.3                                                   ______________________________________                                    

A total of 9.3 g of catalyst A of Example 1 was regenerated anddispersed to give catalyst G, using the procedure described in Example 1except that the flow rates, conditions and steps given below were used:

    ______________________________________                                                           Gas                                                                   Temp.   Composition                                                                              Flow Rate                                                                            Duration                                 Step       (°C.)                                                                          (%)        (cc/min.)                                                                            (hrs.)                                   ______________________________________                                        Dry H.sub.2 Treat                                                                        510     20% H.sub.2                                                                              840    1.5                                      Dry HCl Treat                                                                            510     0.21% HCl  300    2.5                                      Dry        510     1.8% O.sub.2,                                                                            300    2.25                                     Oxychlorination    0.21% HCl                                                  Dry        510     7.2% O.sub.2,                                                                            300    6.8                                      Oxychlorination    0.21% HCl                                                  Dry H.sub.2                                                                              510     20% H.sub.2                                                                              500    1.5                                      Reduction                                                                     ______________________________________                                    

This example indicates that the coke burn step can be omitted whenoxygen is present during chlorination.

The ECAT results using a feed of 20% methylcyclopentane and 80% n-hexaneshow that catalyst G had poor initial activity and activity maintenance.Thus, an oxygen post-treat step is required.

    ______________________________________                                                     Benzene Yield (wt. %)                                                         Time on Oil (hrs.)                                               Catalyst       2 hrs.  22-24 hrs.                                             ______________________________________                                        Catalyst G     7.4     4.1                                                    Catalyst B     14.4    12.3                                                   ______________________________________                                    

In summary, the present invention is seen to provide a method ofregenerating moderately or severely deactivated reforming catalysts,preferably potassium-exchanged zeolite L containing platinum, therebysubstantially restoring catalyst activity and activity maintenance tothe catalyst.

What is claimed is:
 1. A process for treating a deactivatedmonofunctional, non-acidic reforming catalyst comprising L zeolite, atleast one Group VIII noble metal, an inorganic binder, and carbondeposits, said process comprising:a) contacting said catalyst with agaseous stream comprising inert gas comprising up to about 10 vol. %water at a temperature within the range of about 380° C. to about 540°C. for up to about 10 hours; b) subjecting said catalyst to a gaseousstream comprising inert gas, 0.1 vol. % to 25 vol. % oxygen, less thanabout 10 vol. % water, and about 0.005 vol. % to about 10 vol. % of asource of chlorine under conditions comprising a temperature within therange of about 400° C. to about 530° C. effective for dispersing the atleast one Group VIII noble metal on the surface of said catalyst; and c)treating said catalyst with another gas stream comprising a gascomprising less than about 10 vol. % water and from about 0.1 vol. % toabout 25 vol. % oxygen, said gas being selected from the group of gasesconsisting of oxygen, steam, and mixtures of oxygen and steam, underconditions comprising a temperature within the range of about 400° C. toabout 540° C. effective to remove excess chlorine from the zeolitecatalyst, stabilize the Group VIII noble metal, and result in said GroupVIII noble metal being well dispersed over said surface of said catalystupon subsequent reduction; and d) contacting said catalyst from whichexcess chlorine has been removed with another gaseous stream comprisingless than about 10 vol. % water, inert gas, and about 1 vol. % to about25 vol. % hydrogen, at a temperature within the range of about 400° C.to about 530° C. for up to about 10 hours to effect a reduction of saidGroup VIII noble metal particles of said catalyst.
 2. The process fortreating said catalyst of claim 1, wherein said process furthercomprises contacting said deactivated catalyst after step a) and priorto step b) with a gas stream comprising inert gas, less than about 10vol. % water, about 0.1 to about 25% hydrogen, and about 0.05 to about4% of a source of chlorine at a temperature within the range of about400° C. to about 530° C. for up to about ten hours to reduce saidcatalyst and treat said reduced catalyst with hydrogen chloridegenerated by the hydrogen and the source of chlorine.
 3. The process ofclaim 1, wherein at least about 90% of said particles have a diameter ofless than about 7 Angstrom as measured with bright field imaging on atransmission electron microscope with 4 Angstrom point-to-pointresolution.
 4. The process of claim 1, wherein said gas stream in stepa) is substantially devoid of water.
 5. The process of claim 1, whereinthe source of chlorine of said gaseous stream of step b) is a memberselected from the group consisting of Cl₂, HCl, chlorinated hydrocarbonsand mixtures thereof.
 6. The process of claim 5, wherein said source ofchlorine of said gaseous stream of step b) is a member selected from thegroup consisting of HCl and Cl₂.
 7. The process of claim 1, wherein saidgas in step c) comprises at least one member selected from the groupconsisting of oxygen, and mixtures of oxygen and steam.
 8. The processof claim 7, wherein said gas in step c) comprises a mixture of oxygenand steam.
 9. The process of claim 1, wherein said amount of oxygen ofsaid gas in step c) is within the range of from about 0.5 vol. % toabout 12 vol. %.
 10. The process of claim 1, wherein said amount ofwater in said gas in step c) is within the range of from about 1.0 vol.% to 3.0 vol. %.
 11. The process of claim 1, wherein said gas in step c)contains essentially no water.
 12. The process of claim 1, wherein saidstep c) is continued for a time less than about 7 hours.
 13. The processof claim 1, wherein said source of chlorine in said gaseous stream ispresent in said gaseous stream in step b) is present within the range ofabout 0.05 vol. % to about 4 vol. %.
 14. The process of claim 1, whereinsaid another gaseous stream in step d) contains essentially no water.15. The process of claim 1, wherein said another gaseous stream in stepd) comprises about 0.5 vol. % to about 5 vol. % water.
 16. The processof claim 1, wherein said inert gas is nitrogen.
 17. The process of claim1, wherein said inorganic binder material is selected from the groupconsisting of kaolin, alumina and silica.
 18. The process of claim 1,wherein said L zeolite comprises exchangeable cations at least a portionof which are selected from the group of exchangeable cations consistingof lithium, sodium, potassium, rubidium, cesium, calcium, and barium,and mixtures of lithium, sodium, potassium, rubidium, cesium, calcium,and barium.
 19. The process of claim 18, wherein said exchangeablecations are selected from the group consisting of potassium, and barium.20. The process of claim 19, wherein said Group VIII noble metal isplatinum.
 21. The process of claim 1, wherein said catalyst metalcomprises rhenium.
 22. The process of claim 1, wherein the amount ofsaid Group VIII noble metal present is within the range of about 0.2 wt.% to about 6 wt. %.